Referring to FIG. 1, an electro-magnetic interference (EMI) image plane 110 according to the conventional art is shown. The EMI image plane acts to suppress EMI emission from a source such as a printed circuit assembly (PCA) 120. As depicted in FIG. 1, the EMI image plane 110 comprises a conductive layer orientated in close proximity to the PCA 120. Furthermore, the EMI image plane 110 is electrically isolated from the PCA 120. In one implementation, the EMI image plane 110 is coupled to the PCA 120 by a plurality of insulating spacer hardware 130.
The EMI image plane 110 works by creating an image of local noise currents that are opposite to source noise currents in the PCA. The absence of a ground connection between the EMI image plane 110 and the PCA 120 allows image currents to remain local to their source energy and provide effective cancellation of magnetic flux. Thus, the net noise energy, radiating from a PCA 120 is reduced.
The image plane provides relatively good suppression of EMI energy radiated by the PCA 120 beyond the EMI image plane 110. However, the EMI image plane 110 can acquire a static charge, relative to the PCA 120, because it is electrically isolated from the PCA 120. When the static charge builds up to a sufficient level it typically discharges through the PCA 120 because of its close proximity thereto. Such a discharge regularly causes damage to the electrical components on the PCA 120.
Referring to FIG. 2, an electro-magnetic interference (EMI) shield 210 according to the conventional art is shown. The EMI shield 210 acts to suppress EMI emission from a source, such as a printed circuit assembly (PCA) 220. As depicted in FIG. 2, the EMI shield 210 is orientated in close proximity to the PCA 220 by a plurality of spacer hardware 330, 331. In one implementation, one of the spacers 331 is electrically conductive and is utilized to couple the EMI shield to a ground 225 of the PCA 220. The connection 331 to ground 225 provides a low impedance path.
The EMI shield 210 works by draining the EMI energy radiated by the PCA 220 to ground. Furthermore, the EMI shield 210 does not acquire a static charge because it is grounded to the PCA 220. Thus, damage resulting from electrostatic discharge is reduced. However, the EMI shield 210 provides relatively poor suppression of EMI radiation (with respect to an EMI image plane).
Referring now to FIG. 3 a cover 310 according to the conventional art is shown. As depicted in FIG. 3, the cover 310 is non-conductive and orientated in close proximity to the PCA 320. In one implementation, the cover 310 is coupled to the PCA 320 by a plurality of insulating spacer hardware 330.
The cover 310 may comprise a front-side (component) cover and/or a backside (solder-side) cover. The cover 310 is utilized to prevent mechanical and/or electrical damage to assemblies and/or components due to electrical shorting, electrostatic discharge, improper insertion or extraction, as well as during handling, and regular PCA operation. Furthermore, the use of one or more covers 310 substantially increases the dimensions of the assembly.
Thus, a need exists for an improved method of reducing EMI emission from a source. The method and apparatus should provide protection to adjacent devices from electro-static discharge, shorting, mechanical damage and the like. The method and apparatus should also be relatively inexpensive. The method and apparatus should also be relatively compact. The present invention provides a novel solution to the above needs.